Ion implanters are commonly used in the production of semiconductor wafers. An ion source is used to create an ion beam, which is then directed toward the wafer. As the ions strike the wafer, they dope a particular region of the wafer. The configuration of doped regions defines their functionality, and through the use of conductive interconnects, these wafers can be transformed into complex circuits.
A block diagram of a representative ion implanter 100 is shown in FIG. 1. An ion source 110 generates ions of a desired species. In some embodiments, these species are atomic ions, which are best suited for high implant energies. In other embodiments, these species are molecular ions, which are better suited for low implant energies. These ions are formed into a beam, which then passes through a source filter 120. The source filter is preferably located near the ion source. The ions within the beam are accelerated/decelerated in column 130 to the desired energy level. A mass analyzer magnet 140, having an aperture 145, is used to remove unwanted components from the ion beam, resulting in an ion beam 150 having the desired energy and mass characteristics passing through resolving aperture 145.
In certain embodiments, the ion beam 150 is a spot beam. In this scenario, the ion beam passes through a scanner 160, which can be either an electrostatic or magnetic scanner, which deflects the ion beam 150 to produce a scanned beam 155-157. In certain embodiments, the scanner 160 comprises separated scan plates in communication with a scan generator. The scan generator creates a scan voltage waveform, such as a sine, sawtooth or triangle waveform having amplitude and frequency components, which is applied to the scan plates. In a preferred embodiment, the scanning waveform is typically very close to being a triangle wave (constant slope), so as to leave the scanned beam at every position for nearly the same amount of time. Deviations from the triangle are used to make the beam uniform. The resultant electric field causes the ion beam to diverge as shown in FIG. 1.
In an alternate embodiment, the ion beam 150 is a ribbon beam. In such an embodiment, there is no need for a scanner, so the ribbon beam is already properly shaped.
An angle corrector 170 is adapted to deflect the divergent ion beamlets 155-157 into a set of beamlets having substantially parallel trajectories. Preferably, the angle corrector 170 comprises a magnet coil and magnetic pole pieces that are spaced apart to form a gap, through which the ion beamlets pass. The coil is energized so as to create a magnetic field within the gap, which deflects the ion beamlets in accordance with the strength and direction of the applied magnetic field. The magnetic field is adjusted by varying the current through the magnet coil. Alternatively, other structures, such as parallelizing lenses, can also be utilized to perform this function.
Following the angle corrector 170, the scanned beam is targeted toward the workpiece 175. The workpiece is attached to a workpiece support. The workpiece support provides a variety of degrees of movement.
A traditional ion source is shown in FIG. 2. A chamber housing 10 defines an ion chamber 14. One side of the chamber housing 10 has an extraction aperture 12 through which the ions pass.
A cathode 20 is located on one end of the ion chamber 14. A filament 30 is positioned in close proximity to the cathode 20, outside of the ion chamber. A repeller 60 is located on the opposite end of the ion chamber 14.
The filament 30 is energized by filament power supply 54. The current passing through the filament 30 heats it sufficiently (i.e. above 2000° C.) so as to produce thermo-electrons. A bias power supply 52 is used to bias the cathode 20 at a substantially more positive voltage than the filament 30. The effect of this large difference in voltage is to cause the thermo-electrons emitted from the filament to be accelerated toward the cathode. As these electrons bombard the cathode, the cathode heats significantly, often to temperatures over 2000° C. The cathode, which is referred to as an indirectly heated cathode (IHC), then emits thermo-electrons into the ion chamber 14.
The arc supply 50 is used to bias the ion chamber housing 10 positively as compared to the cathode. The arc supply typically biases the housing 10 to a voltage about 50-100 Volts more positive than the cathode 20. This difference in voltage causes the electrons emitted from the cathode 20 to be accelerated toward the housing 10.
A magnetic field is preferably created in the direction 62, typically by using magnetic poles located outside the chamber. The effect of the magnetic field is to confine the emitted electrons within magnetic field lines. A second effect is to cause the electrons to move from the cathode toward the opposite end of the chamber in a spiraling fashion (as shown in FIG. 3).
Vapor or gas source 40 is used to provide atoms or molecules into the chamber 14. The molecules can be of a variety of species, including but not limited to inert gases (such as argon), hydrogen-containing gases (such as PH3 and AsH3) and other dopant-containing gases (such as BF3). The temperature of the chamber, coupled with the emitted electrons traveling in the chamber 14 serve to transform this injected gas into a plasma.
At the far end of the chamber, opposite the cathode 20, a repeller 60 is preferably biased to the same voltage as the cathode 20. This causes the emitted electrons to be repelled away from the repeller 60 and back toward the cathode 20. The use of these like-biased structures at each end of the chamber 14 maximizes the interaction of the emitted electrons with the material (i.e. gas and plasma) that exists in the ion source chamber. The result of these interactions between the emitted electrons and the gas and plasma is the creation of ions.
FIG. 3 shows a different view of the ion source of FIG. 2. The magnet 86 creates a magnetic field 62 across the ion chamber. The cathode 20 and repeller 60 are maintained at the same potential, so as to repel the electrons away and toward the center of the chamber. When the gas interacts with the electrons, plasma 80 is created. An electrode 90 is biased to attract the ions through the extraction aperture 12. These extracted ions form an ion beam 95 and are used as described above.
The temperatures, corrosive gasses and emitted electrons create a harsh environment within the ion source. In fact, the conditions are such that tungsten parts that make up the housing are damaged within days. As a result, some ion source manufacturers have developed liners that can be inserted into the ion source. These liners cover the side and bottom surfaces of the ion source housing, thereby protecting the housing 10 from this harsh environment. These liners are typically made using tungsten or molybdenum and are simply slid into the housing. These liners are still damaged by the environment in the ion source; however, since these liners are typically much less costly than the ion source housing itself, it is cost effective to insert them into an ion source housing to prolong the useful life of the ion source housing. When the liners wear out, they are simply discarded and replaced with new ones.
These liners perform an important function in prolonging the life of an ion source and reducing the annual operating cost. In addition, it would also be beneficial if the ion source liner were also able to improve the power efficiency of the ion source, such that either more ions can be produced at a given power level, or an equal number of ions can be produced at a lower power level.